Data communication standards such as RS485 and RS232 have been used for a long time and have become well established. Consequently there is a continuing desire to be able to exchange data in conformity with these standards. The RS485 standard requires that a bus driver be able to drive a differential signal of at least 1.5 volts across a 54 ohm load. The RS 422 standard requires a bus driver to be able to supply a 2 volt single ended signal across a 100 ohm load.
Bus drivers may be connected in parallel to a shared bus. Therefore each driver output stage needs to be able to be placed in a high impedance state if the connected bus drivers are not to fight one another. This condition, when multiple drivers are connected to a shared bus and are active at the same time is known as “bus contention”. Drivers which are able to be placed into a high impedance state are commonly known as “tri-state drivers”.
Additionally, bus drivers must also be robust against the risk that a given bus driver is not powered up, but that a voltage has been applied to a bus to which the driver is connected. Under such conditions the driver must not exit from a high impedance state and must not open an uncontrolled current flow path between the bus and the power supply rails of the bus driver or between the bus and the signal path to the components that drive the bus driver. Furthermore the bus driver must be robust against overvoltage events that might occur from switching off inductive loads or the like in, for example, industrial, avionic, nautical or automotive environments.